Air Pollution Control Innovations

Venturi scrubbers are used to remove particulate from the exhaust gas of industrial sources.  They are highly efficient at removing particulate 1-micron in size and larger. Venturi scrubbers are used in solid waste incineration, waste-to-energy production, mining, biosolids sludge processing, plastics production and coal gasification.  In many of these applications, the Venturi is used on the back end of a dryer or thermal destruction device.

In accordance with Bernoulli's equation, inlet gas accelerates at the converging section, increasing gas-liquid contact. As water is injected perpendicular to the gas flow, the accelerated gas particles are captured by water droplets upon collision. The resulting droplets aggregate through the diverging section and are separated from the process gas by the mist eliminator (ME) in the entrainment separator (ES). 

A secondary lead smelter in Southern California recovers lead from used automobile batteries. The process uses a combination of combustion processes including kilns and furnaces. Despite extensive control equipment including bag-houses, HEPA filters, and wet scrubbers, low concentrations of heavy metals were being emitted from various stack sources.  Cancer risk index standards were being exceeded due to the close proximity of residents to the plant.  To be compliant with state regulations a solution was needed to reduce these emissions. The special circumstances of nearby residents and a stringent cancer risk index required the facility to meet emission limits well below any other secondary lead smelter in the country.

A case study is now available to describe how an Envitech high performance Wet Electrostatic Precipitator was used to solve this problem.

Please click on the icon below to download the case study.

A specialty cellulose provider, Tembec, is upgrading a sulfite pulping mill in Eastern Canada to incorporate a new red liquor recovery boiler for power generation.  The boiler system increases annual green energy production by up to 40-megawatts, reduces sulfur dioxide emissions by 70%, and increases annual production capacity of specialty cellulose by 5,000 metric tonnes.  The off-gas from the recovery boiler contains a high concentration of SO2 which needs to be recovered for re-use in the production process. 

A cost effective solution was needed to recover sulfur and clean the stack gases to meet stringent particulate and SO2 emission limits.  The plant is located in a high labor rate area, which makes a field erected system very expensive due to lengthy installation time.

A case study is now available to describe how an Envitech scrubber/wet electrostatic precipitator (WESP) system solves this problem.

Please click on the icon below to download the case study.

I previously posted a blog post about a new project for lead removal using a wet electrostatic precipitator (WESP). A Midwestern manufacturer heat treats metal materials in a furnace that generates low concentrations of lead fumes.  The fumes are sub-micron in size, making them difficult to remove.  The facility is compliant with their existing permit, but wants to be pro-active to control emissions further. They investigated several technologies including HEPA and cartridge filters, Venturi scrubbers, and wet electrostatic precipitators (WESPs). Because of the small particle size, low concentration, and high removal efficiency, they had difficulty finding technologies or vendors willing to guarantee the requested performance limit.

A case study is now available to describe how an Envitech high performance Wet Electrostatic Precipitator solves this problem.

Please click on the icon below to download the case study.

Envitech has received an order from Tembec to supply an ammonia based red liquor recovery boiler scrubber for the Temiscaming sulfite pulping mill in Québec, Canada.   The scrubber system will treat the off-gas from an upstream sulfite power boiler supplied by Andritz and will meet stringent emission standards for particulate and SO₂.

This system is part of a $190-million power generation project that will upgrade Tembec’s existing specialty cellulose manufacturing facility.  The upgrade will increase annual green energy production by up to 40-megawatts, reduce sulfur dioxide emissions by 70%, and increase annual production capacity of specialty cellulose by 5,000 metric tonnes.  The upgrade will make Temiscaming one of the lowest-cost specialty cellulose manufacturing facilities in the world.

The scrubber system is engineered to Tembec’s requirements and is the result of close collaboration with the customer over several months and their engineering firm Pöyry.   The design leverages experience from several other large volumetric flow rate applications including a coal dryer scrubber, secondary lead smelter wet electrostatic precipitator (WESP) system, and a hazardous waste incinerator scrubber.  The scope of supply includes an inlet duct from the boiler exhaust fan, quencher, ammonia scrubber, condenser, WESP, caustic scrubber, heat exchangers, structural steel, access platforms, and stack.  Primary vessels will be shop fabricated and shipped to the site for assembly and installation.  This provides a lower total installed cost, greater quality control over manufacturing, and shorter delivery and installation time.  System start-up will occur in the fall of 2013.

To read more about this system, download the case study below.

A common application for Venturi scrubbers is potash mining.  Venturi scrubbers are used to remove particulate from industrial dryers that dry the potash material.  A cyclonic separator or chevron style entrainment separatordownstream of the Venturi removes particulate–laden water droplets from the gas before it is exhausted into the atmosphere.  The type of entrainment separator can impact the system performance and make a significant difference in plant emissions.  A well designed chevron style mist eliminator achieves significantly lower emissions than a cyclonic separator.  This is because the chevron style mist eliminator allows the Venturi scrubber to operated at a higher pressure drop for the same system pressure drop. The chevron entrainment separator is also more efficient at removing water drops.

A chevron style mist eliminator removes more than 99% of the water droplets down to 25 microns with less than 0.25 inches W.C. of pressure drop.  By comparison, a cyclonic mist eliminator requires 4 to 6 inches W.C. of pressure drop to remove 98% of 25 micron droplets.  The additional pressure drop required by the cyclonic separator effectively reduces the pressure drop that can be applied to the Venturi throat and consequently lowers collection efficiency.  The additional water droplets that escape the cyclonic separator (98% vs. 99% removal for 25 micron water droplets) contain particulate which will impact stack test results.

Consider an air permit for a potash facility located in the Southwestern United States.  The permit allows for the operation of two 2 dryer systems, each with a Venturi scrubber. The dryers contribute significantly to the facility wide total suspended particulate (TSP) emissions.  A typical potash dryer for this size of facility exhausts around 40,000 acfm with a particulate loading as high as 3 gr/dscf.  This is a little more than half a ton per hour of particulate for both dryers.

Venturi scrubbers with cyclonic separators designed for 99% removal result in approximately 10.4 lb/hr of particulate emissions for both scrubbers or 46 tpy. However, an equivalent Venturi scrubber with a chevron style mist eliminator achieves 99.9% removal. This results in less than 1 lb/hr of particulate emissions for both dryers, or less than 5 tpy. The higher performing Venturi scrubber design reduces plant emissions by over 40 tpy. This performance increase can have a significant difference in air quality for the surrounding community.

To download a case study on dryer Venturi scrubber, please click on the download button below.

I’ve made several blog posts regarding the removal of hazardous air pollutant (HAPs), including lead, from a secondary lead smelter in CA using a Wet Electrostatic Precipitator (WESP). There is also an article published in the August 2010 issue of Pollution Engineering on this system.  My last blog post references a letter to the EPA reporting metals reductions as high as 98% and 99% and reductions of lead from 615 lb/yr to 1.22 lb/hr.  The facility is now achieving emissions that are several orders of magnitude lower than other secondary lead smelters.

Envitech is in the design phase for a new project that will apply the WESP technology to an industrial facility that has lead emissions similar to stack emissions from a controlled secondary lead facility.  The WESP system guarantees 95% lead removal from inlet concentrations of 0.003 gr/dscf.

The system was selected after the facility performed a thorough evaluation of different technologies including HEPA and cartridge filters as well as other WESP suppliers.   The system will be operational in Q1 2013 with stack test results due by the middle of 2013.

Envitech is implementing cost reductions that will enable reduced capital expenditure for future installations and make the technology more economically feasible to be considered best available control technology (BACT) by the EPA.   This new system will be a good surrogate to further demonstrate the performance of the Envitech WESP system to get the lead out!

Click on the link below to download a white paper from the International Conference on Thermal Treatment Technologies and Hazardous Waste Combustions (IT3/HWC) on a WESP system for a secondary lead smelter.

In 2009 I gave a paper at International Conference on Thermal Treatment Technologies (IT3) on a wet electrostatic precipitator (WESP) system for the Quemetco secondary lead smelting facility in Southern California.  It was explained that the system was an add-on control to help the plant meet a cancer risk index by removing low concentrations of metals.  The facility was meeting stack emission limits and had similar control technology used by other facilities in the industry including bag-houses, HEPA filters, and wet scrubbers. Because residential neighborhoods had moved closer to the boundaries of the plant over the years, the stack emission limits were insufficient to meet the cancer risk index.  In other words, the facility had to achieve lower emission levels than other similar plants. The WESP system was installed in 2007 and provided a performance guarantee for arsenic, lead, and nickel which were the larger contributors to the cancer risk index.

In May 2011, the EPA released the new National Emissions Standards for Hazardous Air Pollutants (NESHAP) for secondary lead smelting facilities.  The report sites in section III.B.2 that two of the 14 secondary lead smelting facilities have estimated actual lead emission only moderately lower than the allowable emission level (about 2-3 time lower).  The majority of the other facilities have estimated actual emissions in the range of 10 to 100 times lower than allowable.  However, one facility with highly advanced controls has an estimated actual emission of about 1,500 times below allowable emission levels. This facility is the Quemetco facility operating with a WESP system.  It can be surmised the plant is achieving emission levels 15 to 150 times lower than the other lead smelting facilities.

In June of 2010 a public letter on behalf of RSR Corporation (Quemetco) was presented to Mr. Charles French, Metals and Minerals Group, of the EPA. The letter states that the WESP system yielded dramatic reductions in air emissions from its operations.  The cancer risk was reduced by 87% to 2.88 cancer cases in one million exposed individuals over a 70-year evaluation period.  Prior to installation of the WESP, Quemetco emitted 615 pound of lead annually.  The letter reports that lead emissions plummeted to 1.22 pounds per year – a 99.8 percent reduction.  A table is provided in the letter that summarizes significant reductions of other hazardous air pollutants. Some of the reported metals reductions include the following: 

The data presented in the letter is consistent with the initial stack test data that was shared with Envitech following the installation.  Although the system provided a guarantee for only three metal compounds, we expected similar removal for any condensed metal at the inlet of the WESP.  The data demonstrates this was the case.

The consistent performance since the installation in 2007 demonstrates the tremendous capability of the WESP system to achieve dramatic reductions of condensed metals in a robust fashion for a process that must operate 24/7/365 days per year.

For a copy of the full paper, please download the white paper below.

In previous blog posts, I have discussed how a key to particle collection in a Venturi scrubber is maintaining uniform water distribution across the Venturi throat to collide with particles.  This presents a Venturi scrubber design challenge for large volumetric gas flow rate processes.  The difficulty becomes getting water across a large cross sectional area  without any void spaces for particles to escape through.  Often times, the solution may be to simply split the gas flow into multiple trains.  However, this increases capital costs for additional ductwork and piping and takes up more real estate. It is always desirable to minimize the equipment footprint and maintain the gas flow in one train.

To achieve this objective, Envitech uses a proprietary Venturi throat design that has been used on large gas flow rates processes, including foundries and purified teraphthalic acid (PTA) plants. The proprietary design has an internal construction that ensures uniform water distribution throughout the Venturi throat cross sectional.

The adjacent image shows a picture of an Envitech Venturi/Quencher constructed from Hastelloy C276 used for a PTA plant with a gas flow rate of 530,000 acfm.  This type of Venturi design may be used on other large gas flow rate processes like a coal dryer system for a coal gasification plant which can have a gas flow rate as large as 300,000 to 400,000 acfm. 

For another large flow Venturi application, read our case study on particulate removal for a coal dryer.

Last November I made a blog post describing the use of a wet electrostatic precipitator (WESP) for meeting metals emisssions.  This topic will be discussed in greater detail in an Envitech paper being presented at the 2010 A&WMA 103rd annual conference in Calgary, Canada.  

The metals of concern can include mercury, arsenic, lead, cadmium, nickel and others, depending on the process.  A control strategy using a wet electrostatic precipitator (WESP) in conjunction with sub-cooling was used on a secondary lead smelter to meet more stringent emission standards.  This approach achieved > 98% removal of arsenic and > 92% removal of lead and other condensed metals downstream of a bag-house.  This substantially reduced the plant's cancer risk index and helped to meet reduced fence line lead emission limits.   

The table below provides a summary of other processes facing similar challenges including the additional removal efficiencies that can be required downstream of existing air pollution controls.  Most of the processes are from combustion sources and use a range of air pollution controls including bag-houses, packed bed absorbers, and Venturi scrubbers.  In some cases a combination of controls are used. Despite existing controls, very low concentrations of heavy metals can be emitted.  In the case of bag-houses for instance, the operating temperature may be in a range that some of the metals are in a gas phase.  In such case they will not be collected by particulate control devices. In other cases, the concentrations of submicron, condensed phase heavy metals may exceed the removal capability of controls like packed bed absorbers or Venturi scrubbers.

*Refers to additional removal efficiency after the upstream controls. 

The performances achieved on a secondary lead smelter using a WESP, suggest the approach can be used on these other processes to remove residual concentrations of condensed metals.  In the case of mercury, the ability to remove it with a WESP depends on whether it is in a condensed form. This requires reliable speciation data to make that determination.  More specifically, the mercury must be in a particulate or oxidized form for it to be removed by a WESP.

Please click on the below icon to download a white paper on this topic from the 2010 A&WMA's 103 Annual Converence in Calgary, Canada: "Wet Electrostatic Precipitator (WESP) Control for Meeting Metals Emission Standards".